Alternating current rectifier



Patented May 21, 1940 v 2,201,940 ALTEBNATING cnaann'r ancrmma Etienne Barrey, Nanterre, Seine, France, aslignor. of fifty per cent to Maurice Joseph Louis Masnou, Paris, France No Drawing. Application October 6, 1939, Serial No. 298,335. In France October '7, 1938 14 Claims. (01. 115-366) The present invention relates to an improvement in alternating current rectifiers, of the type consisting in a copper plate or washer covered with a layer of crystallised red copper oxide formed on the said plate or washed by oxidation in an oven at a high temperature. It is known that after they have been oxidised, these plates or washers are tempered either directly after said oxidation, or after a gradual cooling in down to a certain temperature intermediate between the oven and the-tempering temperatures, and thereafter treated by suitable mechanical, chemical or electrochemical processes with the view of eliminating or reducing the superficial ll layer of black oxide which is formed during the treatment in the oven and which presents an unduly high ohmic resistance.

It is also known that the rectifiers manufactured in this manner are very non-uniform and give rise to a large amount of scrap, even if great care is taken in their manufacture.

It is also known that the copper used for the manufacture of these, rectifiers contains a certain proportion of other metals such as iron,

nickel, lead and tin, so that during the oxidation process of the copper plates there is ob-' tained a complex superficial layer constituted by copper oxide and oxides of these other metals. It has beenfound that the presence of these foreign oxides is detrimental to the satisfactory operation of the rectifier, so that any incorporation to the copper, before its oxidation, of another metal, especially a white metal (tin, zinc, magnesium) should be carefully avoided, it being desirable to use copper in the highest state of pm'ity technically possible.

It has also been proposed, in the case where the rectifying cells were used as photoelectric cells, to interpose between the copper and the 40 copper oxide a fine layer of a precious metal such as gold, but such an arrangement will exert no advantageous eflects from the point of view of rectification.

Rectifiers are known on the other hand, consisting of two electrodes of different metal, ar-

ranged adjacent to one another and having for instance the shape of discs or plates, a rectifying film being formed upon the contact surface of one of these electrodes. This electrode may consist for instance in an alloy of copper with zinc or tin, while the rectifying film is formed by combining this metal or alloy with oxygen, sulphur, selenium or tellurium, in a manner tocreate a complex superficial layer of oxides, l sulphides, selenides, or tellurides, which is pressed against the second electrode of a copper or white metal base metal or alloy. These rectifiers, which, moreover, are of a type different from that with which the present invention is concemed, may however only be used for com- I paratively low power outputs and their life is fairly short, while the rectifiers manufactured as described in the present specification have a practically unlimited life.

The rectifier forming the subject of the present 10 invention is of the type comprising a body of copper, on the surface of which is formed a layer of copper oxide crystals in intimate contact with the copper, and it is characterised by the fact that it comprises a white metal, such as magll nesium, zinc or tin, dispersed in the copper oxide layer and adsorbed by the crystals forming this layer.

It should be quite specially pointed out that according to the invention the white metal or a metals are incorporated in the copper oxide layer inmetal form and not in the form of oxides, in contradistinction to the case of certain known rectifiers, in which a white metal is incorporated in the copper before any treatment, 25 in such a manner that thereafter there is formed a complex layer of oxides or salts of copper and white metals.

The inventor has found that while the presence of oxides of white metals in the copper 3o oxide layer is useless or even detrimental, on the contrary the presence of particles of white metals themselves, dispersed in this layer and adsorbed by the copper oxide crystals is advantageous from the following three points of view: 35

firstly it will improve the operation of the rectifier by decreasing the reverse current .and increasing the rating of the rectifier; secondly, as will be explained below, the presence of these white metals in the black oxide formed dur- 4o ing the oxidation of the copper discs or plates in the oven permits a more easy reduction of this black oxide layer; and lastly these rectifiers are more uniform in manufacture and give rise to a smaller amount of scrap. 45

In order to realise this adsorption of the white metal by the crystals of copper oxide, the copper plates are treated, according to the invention, during the oxidation process, by vapours of white metals. For this purpose, the oxidation 50 oven may, for example be brought to a temperature of between 1040 and 1080 0., preferably as near as possible to the latter temperature, while a piece of the white metal or of an alloy of white metals is introduced into the oven together with the copper plates to be oxidised. At the high oven temperature, the white metal is volatilised and the metal vapours thus produced will deposit on the copper plates where they are adsorbed by the copper oxide crystals which are just being formed. In order to regularise this sort of dispersion of the white metal in the copper oxide, an electric potential difference is preferably applied between the pieces of whitemetal and the copper plates which are undergoing oxidation, the current set up in this way through the atmosphere containing the conducting metal vapour of the oven having the effect of directing the metal ions upon the copper washers and distributing them evenly thereupon. The white metals are introduced into the oven preferably in the form of a magnesium and a zinc wire twisted together. The layer which is thus formed on the surface of the copper plate and which comprises red copper oxide crystals and particles of adsorbed white metals, has the effect of creating a unidirectional contact having a very low ohmic resistance in the direction of the flow of current, the thus obtained decrease of the ohmic resistance may be in the ratio of up to 5 to 1 as compared with the copper oxide rectifiers of the usual type. Furthermore, the rectifiers obtained in this way will present, as indicated above, a very high regularity.

Another way of causing the adsorption of a white metal by the copper oxide crystals of the rectifying cells consists in immersing oxidised plates coming out of an oven in a bath of a nonoxidising substance such as graphite and/or copper oxide in powder form, raised to a very high temperature, near to that of the oxidising oven, and containing in addition a white metal such for instance as tin. The white metals and copper having at high temperatures a very great affinity for one another, the white metal particles or drops dispersed in the graphite in contact with the copper plates immersed in the bath will penetrate immediately into the oxide layer in the molten state covering the plates and will be adsorbed thereafter by the copper oxide crystals formed during the cooling of the plates. The penetration of the white metal particles into the copper oxide layer may be facilitated by operating according to the method described in the U. S. Patent No. 2,139,791 of the present inventor, i. e., passing an electric current through the oxidised plate immersed in the graphite and/or copper oxide bath during the crystallization phase of the copper oxide, the passing of the current further exerting a very favourable effect upon the rectification properties of the element, as has been indicated in Patent No. 2,139,791 referred to above.

The present invention further concerns means permitting the reduction of the superficial blacli copper oxide layer which is formed on the copper plates oxidised in the oven. It is known that this reduction may only be effected with the aid of weak reducing agents in order not to attack the underlying layer of red copper oxide and in order not to reduce the oxide layers on the edges of the plate, which would cause a short-circuiting of the rectifier. The result is that the reduction of the black oxide is difficult and unsatisfactory.

The inventor has found that the reduction of the black oxide layer by the weak reducing agents is greatly facilitated in the case of a rectifier manufactured according to the process described above, 1. e. when a white metal, especially tin, is incorporated to said black oxide. and more particularly when the tin is incorporated in a fairly large amount by immersing the plates in a bath of graphite and/or copper oxide containing an addition of tin and raised to a high temperature. When the copper plates have been treated during their oxidation by vapours of the white metals, the proportion of white metal incorporated in the superficial black oxide layer may be increased by an additional treatment according to the second method described above.

The rectifiers obtained in this way are treated, according to the invention, for reducing the black oxide layer, by immersing the oxidised plates or soaking their surface with a colloidal suspension in water of iron, consisting of very small particles (micella) of iron, each grouping only a few molecules. Whilst the plate is immersed in this suspension, or its surface is soaked with it, there is applied to the surface of the black oxide for the purpose of reduction, 9. piece of iron (for instance a fine iron wire brush) connected to the positive terminal of a source of current, for instance a 2 to 6 volt storage battery, the negative terminal of which is connected to the copper of-the plate. Under the effect of the forces set up by the potential drop at the contact between the iron brush and the black oxide surface, the gradient of which drop may attain as much as several tens of thousand volts per centimetre since the total drop is of the order of 2 to 6 volts while the contact between the iron brush and the surface to be reduced may have a width of from one tenth to one hundredth of a millimeter, the iron particles in suspension in the water are projected against the black copper oxide crystals while reducing them completely. On the contrary, the red oxide is not attacked at all. Furthermore, the reduction of the black oxide only takes place at the contact of the iron brush, thus permitting the reduction of only the central part of the plate while preserving the edges.

What I claim is:

1. A dry electric rectifier comprising a copper plate, a copper oxide layer formed on the surface of said plate and a white metal dispersed in the copper oxide layer and adsorbed by the crystals forming said layer.

2. A dry electric rectifier according to claim 1 wherein the white metal is magnesium.

3. A dry electric rectifier according to claim l wherein the white metal is zinc.

4. A dry electric rectifier according to claim 1 wherein the white metal is tin.

5. A dry electric rectifier according to claim 1 wherein an alloy of a number of white metals is dispersed in the copper oxide layer.

6. A method of manufacturing dry electric rectifiers according to claim 1, wherein copper plates are oxidised at high temperature and submitted during their oxidation to the action of the vapour of the white metal to be adsorbed.

7. A method of manufacturing dry electric rectifiers according to claim 1 consisting in oxidising copper plates in an oven brought to a high temperature and introducing into said oven a piece of the white metal to be dispersed in the copper oxide layer.

8. A method of manufacturing dry electric rectifiers according to claim 1 consisting in oxidising copper plates in an even brought to a high temperature and introducing into said oven a piece of the white metal to be dispersed in the copper oxide layer, said piece being in the form a nonoxidising substance having added thereto of wires twisted together of a number of white metals.

9. A method of manufacturing dryelectric rectifiers according to claim 1 consisting in oxidising copper plates in an oven brought to a high temperature, introducing into said oven a piece of the white metal to be dispersed in the copper oxide layer and in applying a difference of potential between the piece of white metal to be adsorbed by the copper oxide layer and the copper plates in course of oxidation.

10. A method of manufacturing dry electric rectifiers according to claim 1 consisting in oxidising copper plates in an oven brought to a temperature between 1040 and 1080 C. and introducing into said oven a piece of the white metal to be dispersed in the copper oxide layer.

11. A method of manufacturing dry electric rectifiers according to claim 1 wherein white metals to be adsorbed by the copper oxide layer are incorporated in the said layer after oxidation of the copper plate.

12. A method of manufacturing dry electric rectifiers according to claim 1 which consists in dipping the oxidised copper plates in a bath of the white metal to be adsorbed by the copper oxide layer and brought to a very high temperature, close to the oxidation temperature of copper.

13. A method of manufacturing dry electric rectifiers according to claim 1 which consists in dipping the oxidised plates in a bath of graphite to which tin has been added and'brought to a very high temperature, close to the oxidation temperature of copper, and in causing said bath of graphite to be traversed by an electric current.

14. A method of manuiacturing dry electric rectifiers comprising a copper plate and a copper oxide layer formed on the surface of said plate, which method consists in incorporating a white metal in said copper oxide layer, then reducing the superficial black copper oxide layer by im- ETIENNE BARREY. 

